Genome editing via sequence-specific nucleases is known. See references 1, 2, and 3 hereby incorporated by reference in their entireties. A nuclease-mediated double-stranded DNA (dsDNA) break in the genome can be repaired by two main mechanisms: Non-Homologous End Joining (NHEJ), which frequently results in the introduction of non-specific insertions and deletions (indels), or homology directed repair (HDR), which incorporates a homologous strand as a repair template. See reference 4 hereby incorporated by reference in its entirety. When a sequence-specific nuclease is delivered along with a homologous donor DNA construct containing the desired mutations, gene targeting efficiencies are increased by 1000-fold compared to just the donor construct alone. See reference 5 hereby incorporated by reference in its entirety. Use of single stranded oligodeoxyribonucleotides (“ssODNs”) as DNA donors has been reported. See references 21 and 22 hereby incorporated by reference in their entireties.
Despite large advances in gene editing tools, many challenges and questions remain regarding the use of custom-engineered nucleases in human induced pluripotent stem cell (“hiPSC”) engineering. First, despite their design simplicity, Transcription Activator-Like Effectors Nucleases (TALENs) target particular DNA sequences with tandem copies of Repeat Variable Diresidue (RVD) domains. See reference 6 hereby incorporated by reference in its entirety. While the modular nature of RVDs simplifies TALEN design, their repetitive sequences complicate methods for synthesizing their DNA constructs (see references 2, 9, and 15-19 hereby incorporated by reference in their entireties) and also impair their use with lentiviral gene delivery vehicles. See reference 13 hereby incorporated by reference in its entirety.
In current practice, NHEJ and HDR are frequently evaluated using separate assays. Mismatch-sensitive endonuclease assays (see reference 14 hereby incorporated by reference in its entirety) are often used for assessing NHEJ, but the quantitative accuracy of this method is variable and the sensitivity is limited to NHEJ frequencies greater than ˜3%. See reference 15 hereby incorporated by reference in its entirety. HDR is frequently assessed by cloning and sequencing, a completely different and often cumbersome procedure. Sensitivity is still an issue because, while high editing frequencies on the order of 50% are frequently reported for some cell types, such as U2OS and K562 (see references 12 and 14 hereby incorporated by reference in their entireties), frequencies are generally lower in hiPSCs. See reference 10 hereby incorporated by reference in its entirety. Recently, high editing frequencies have been reported in hiPSC and hESC using TALENs (see reference 9 hereby incorporated by reference in its entirety), and even higher frequencies with the CRISPR Cas9-gRNA system (see references 16-19 hereby incorporated by reference in their entireties. However, editing rates at different sites appear to vary widely (see reference 17 hereby incorporated by reference in its entirety), and editing is sometimes not detectable at all at some sites (see reference 20 hereby incorporated by reference in its entirety).
Bacterial and archaeal CRISPR-Cas systems rely on short guide RNAs in complex with Cas proteins to direct degradation of complementary sequences present within invading foreign nucleic acid. See Deltcheva, E. et al. CRISPR RNA maturation by trans-encoded small RNA and host factor RNase III. Nature 471, 602-607 (2011); Gasiunas, G., Barrangou, R., Horvath, P. & Siksnys, V. Cas9-crRNA ribonucleoprotein complex mediates specific DNA cleavage for adaptive immunity in bacteria. Proceedings of the National Academy of Sciences of the United States of America 109, E2579-2586 (2012); Jinek, M. et al. A programmable dual-RNA-guided DNA endonuclease in adaptive bacterial immunity. Science 337, 816-821 (2012); Sapranauskas, R. et al. The Streptococcus thermophilus CRISPR/Cas system provides immunity in Escherichia coli. Nucleic acids research 39, 9275-9282 (2011); and Bhaya, D., Davison, M. & Barrangou, R. CRISPR-Cas systems in bacteria and archaea: versatile small RNAs for adaptive defense and regulation. Annual review of genetics 45, 273-297 (2011). A recent in vitro reconstitution of the S. pyogenes type II CRISPR system demonstrated that crRNA (“CRISPR RNA”) fused to a normally trans-encoded tracrRNA (“trans-activating CRISPR RNA”) is sufficient to direct Cas9 protein to sequence-specifically cleave target DNA sequences matching the crRNA. Expressing a gRNA homologous to a target site results in Cas9 recruitment and degradation of the target DNA. See H. Deveau et al., Phage response to CRISPR-encoded resistance in Streptococcus thermophilus. Journal of Bacteriology 190, 1390 (February 2008).